An affiliate is a system that you can join AFTER you have a home park. Coast-2-Coast, AOR, ROD, and RPI are all examples of affiliate systems. You can't belong to them until you have a home park.
Home park can be just an individual park or it can be a system like Thousand Trails, K-M, WHR, SUNRISE, etc. You purchase the home park (or system) either new or used (used is usually the best bet) and then add on the affiliates. It is important though to make sure you understand what the affiliates will cost you. Some home parks (or systems) will charge you are large price to add affiliates if you have purchased a used membership, others the charge is minimal.
As to why to have them - they make it so that not only can you stay at your home park/system, but add HUNDREDS of other parks. We love ROD because it is $0 night for us. But we also use C2C and AOR. In fact we are currently in an ROD affiliated park, we will then go for a wee to a C2C park, then a park under AOR, then back to another ROD park, then a Thousand Trails park, which gets us past Labor Day. Cost for 6 weeks will be $170 or about $4/night.
We average over 100 nights a year in membership parks and have saved thousands of dollars by aggressively using them to our advantage. Sometimes we end up driving a few more miles to visit family/friends, but when the difference is 2 weeks at $0/night versus $40/night, well driving a little is certainly doable!
Barb